Generally, to manufacture a liquid crystal display, a liquid crystal cell containing liquid crystal, and a polarizing film are basically required, and an adhesive layer, or a pressure-sensitive adhesive layer for adhering the two should be used. In addition, a phase retardation film, a compensation film for wide view angle, a brightness enhancement film, etc. may be further adhered to the polarizing film to improve performance of the liquid crystal display.
Typically, a liquid crystal display comprises a uniformly aligned liquid crystal layer; a polarizing film comprising an adhesive layer or a pressure-sensitive adhesive layer; a phase retardation film; and additional functional film layers.
The polarizing film comprises a uniformly aligned iodine compound or dichroic polarizing material. To protect these polarizing elements, such protection film as triacetyl cellulose (TAC), etc. is used to constitute multi layers. The polarizing film may further comprise a phase retardation film having an anisotropic molecular alignment, or a compensation film for wide view angle such as a liquid crystal film.
The aforementioned films are made of materials having different molecular structures and compositions, and so have different physical properties. Especially, under a high temperature and/or humidity condition, the dimensional stability according to shrinking or expanding of materials having anisotropic molecular alignment is insufficient. As a result, if the polarizing film is fixed by a pressure-sensitive adhesive, a shear stress caused by shrinking or expanding of the polarizing film under a high temperature and/or humidity condition remains, whereby light leakage phenomenon occurs at a region on which the stress is concentrated.
To improve the light leakage phenomenon, it is needed to reduce the shrinkage of the polarizing film at a high temperature and/or humidity condition. However, it is very difficult to remove the stress generated from a liquid crystal panel to which a polarizing film consisted of materials having different physical properties is attached. Another method to improve the light leakage phenomenon is to provide stress release function to a pressure-sensitive adhesive layer to fix the polarizer to a liquid crystal panel. Generally, rubbers, acryls, and silicones are commonly used as pressure-sensitive adhesive. Among these, acrylic pressure-sensitive adhesives are advantageous in the pressure-sensitive adhesive property, optical property, durability, and weatherability, and so have been most widely used for manufacturing pressure-sensitive adhesive composition for polarizer.
A general design to provide a pressure-sensitive adhesive layer with stress release function is to design the pressure-sensitive adhesive layer to have large creep property and easy deformability. A representative method thereof is to provide a pressure-sensitive adhesive composition with durability and stress release function under a high temperature and high humidity condition by mixing a low molecular weight polymer having or not having a small amount of crosslinking functional group with a high molecular weight polymer having crosslinking functional group to be able to react with multifunctional crosslinking agent.
For example, Korea Patent Laid-open Publication No. 1998-079266 intended to solve the light leakage phenomenon by providing stress release property to a pressure-sensitive adhesive composition, comprising 100 parts by weight of a high molecular weight acrylic copolymer having a weight-average molecular weight of 1,000,000 or more, 20-200 parts by weight of a low molecular weight acrylic copolymer having a weight-average molecular weight of 30,000 or less, and 0.005-5 parts by weight of a multifunctional crosslinking agent.
Japan Patent Laid-open Publication No. 200247468 intended to provide stress-release function to a pressure-sensitive adhesive composition for polarizing film comprising 100 parts by weight of a high molecular weight acrylic copolymer having a weight-average molecular weight range of from 800,000 to 2,000,000; 5-50 parts by weight of a low molecular weight acrylic copolymer having a weight-average molecular weight of 50,000 or less, and not having functional groups (the degree of dispersion=1.0 to 2.5); a crosslinking agent; and a silane compound.
Also, Japan Patent Laid-open Publication No. 2003-49141 intended to solve the light leakage phenomenon by providing stress release function to a pressure-sensitive adhesive composition for polarizing film comprising a high molecular weight acrylic copolymer having a weight-average molecular weight ranging from 1,000,000 to 2,000,000 and having functional groups; a medium molecular weight acrylic copolymer having a weight-average molecular weight ranging from 30,000 to 300,000 and having less than two functional groups; a low molecular weight acrylic copolymer having a weight-average molecular weight ranging from 1,000 to 20,000 (the degree of dispersion=1.0-2.5) and having no functional groups; and a crosslinking agent.
The above references employ the technical feature to soften a final pressure-sensitive adhesive to improve stress release property of the pressure-sensitive adhesive. That is, the pressure-sensitive adhesive was designed to release local stress resulted from shrinking or expanding of polarizer by increasing the creep property and making the deformation easy against outside stress, as the modulus of pressure-sensitive adhesive is reduced by a low molecular weight material added. However, in case of making a soft pressure-sensitive adhesive by adding a low molecular weight material, the modulus of final pressure-sensitive adhesive is reduced. Then, the pressure-sensitive adhesive may have some problems that it may be easily compressed (pit defect) when a polarizer is stored in role type in the preparation process, and the pressure-sensitive adhesive may be protruded from the cross section of polarizer in case of cutting the pressure-sensitive adhesive, or the polarizer may be contaminated by protruded pressure-sensitive adhesive. Also, the durability of the pressure-sensitive adhesive may be decreased under a high temperature and high humidity condition since the added low molecular weight material may easily migrate into the interface of glass of the liquid crystal panel and TAC.
Another method to provide a pressure-sensitive adhesive layer with stress release function is to maintain the gel content of final pressure-sensitive adhesive prepared as high molecular material having crosslinking functional group very low. This method has an advantage that the modulus of the pressure-sensitive adhesive is not significantly decreased. However, the method also has some disadvantages that it is very difficult to practice the pressure-sensitive adhesive to maintain a uniformly low gel content; it takes a long time from crosslinking of the pressure-sensitive adhesive till cutting of the polarizer (aging time), and the durability is significantly decreased under a high temperature and high humidity condition.
Japan Patent Laid-open Publication No. 60-207101 disclosed a method to reproduce a pressure-sensitive adhesive having low gel content, comprising a high molecular weight material. This Publication disclosed a process for preparing a pressure-sensitive adhesive by mixing an acrylic copolymer (A) having cross-linkable functional group, an acrylic copolymer (B) not having cross-linkable functional group, and multifunctional crosslinking agent having at least two functional groups, wherein the weight ratio of A/B is in the range of 1/4˜4/1. That is, this Publication disclosed a technical feature that the shelf life of the pressure-sensitive adhesive can be extended by removing free cross-linkable functional group through adding a corresponding amount of multifunctional crosslinking agent to the amount of cross-linkable functional group during the crosslinking process. However, this patent did not disclose a molecular weight parameter of acrylic polymer used therein, nor a technical feature of the crosslinking structure. Particularly, the patent did not disclose a technical feature of the stress release property of a pressure-sensitive adhesive related to the light leakage phenomenon.
Thus, there have been needs to develop a new adhesive for polarizing film improving the light leakage phenomenon and preventing the operability decrease of polarizer by minimizing the reduction of modulus of final pressure-sensitive adhesive, without compromising major characteristics of polarizing film products such as durability and reliability in long term usage as well as under a high temperature and humidity condition, and a polarizing film using the same.